Acute viral infections of neurons cause some of the most severe and feared diseases of humans - for instance, rabies, poliomyelitis and, most recently, West Nile virus encephalomyelitis. These diseases are often dramatic in their rapid onset and progression. In those who survive, permanent neurologic deficits often remain. For each of these diseases, neurons are the primary cells that are infected in the central nervous system and both fatal disease and long term sequelae in survivors are caused by virus-induced death of neurons. Remarkable recoveries can also occur, indicating that neuronal infection does not always result in neuronal death. Alphaviruses are important causes of mosquito-borne encephalomyelitis and western, eastern and Venezuelan equine encephalitis viruses are considered category B agents of bioterrorist concern. VEEV has been weaponized. Sindbis virus (SV) is the prototype alphavirus. The outcome of alphavirus encephalomyelitis is determined by properties of both the host and the infecting virus. The process of neuronal cell death is modulated by the types and maturity of the neurons infected, the virulence of the infecting virus and the quality and rapidity of the host immune response. In this application we will focus on the determinants of virus-induced neuronal cell death using SV encephalitis in mice as a model. We have defined amino acid changes in the structural and nonstructural proteins of SV that are important for virulence, have determined that neuronal lipids play an important role in determining susceptibility of neurons to infection, have identified a quantitative trait locus on chromosome 2 that is a primary determinant of genetic susceptibility to infection, have identified glutamate excitotoxicity as an important determinant of neuronal death during infection and have developed an imaging system that allows us to follow virus replication and spread in individual living animals over time. To better understand the viral and host determinants of the outcome from alphavirus infection we propose the following specific aims: (1) To determine the roles of heparan sulfate-binding and N-linked carbohydrates in neurovirulence; (2) To identify the role of neuronal lipids in determining susceptibility of neural cells to infection; (3) To determine the role of lipid signaling in the death of SV-infected neurons; and (4) To determine the role of activation of poly(ADP-ribose) potymerase in the death of SV-infected neurons.